Mechanical seals are seals defined by cooperating first and second structure parts having close fitting planar or curved surfaces. Shaft seals and ball and seat valves are examples of mechanical seal applications. Typically the cooperating parts of mechanical seal structures are formed of metals such as Stellite alloy or like iron, nickel or cobalt base alloys, which may contain chromium, tantalum, molybedenum, titanium, silicon and carbon and the like for the specific properties associated with these elements. Other structures are cermets which comprise non-metallic materials bonded by a metal binder, e.g. such as silicon carbide, the refractory carbides, nitrides, silicides, and borides of metals such as tungsten, tantalum, titanium and zirconium, and a base metal binder of typically cobalt, nickel or iron. The term "nickel base", "cobalt base" or "iron base" herein refers to nickel, iron or cobalt respectively being the largest single metal component present, although such metal may not be a major weight proportion of the structure overall. For example, a cobalt base tungsten carbide structure typically contains about 6% by weight cobalt which acts as the binder in cermet structures made from the tungsten carbide.
Mechanical seals employ a supersmooth interface between opposed cooperating parts, usually one of which rotates or moves linearly relative to the other, the surfaces of the seal structures which engage being lapped smooth to define a fluid seal when in contact, without use of gaskets, seal rings, lubricating films or other foreign expedients. Because the opposed members move relative to one another, maintainence of sealing effect depends on precision of fit initially, and over time on wear resistance. Because there is no lubricating film between the relatively moving seal structures, lubricity of their surfaces is important. Because the constant intimate contact of the members wears one or both of the surfaces, uniformity of wear characteristic is a significant factor in effective mechanical seal design.
Controlling the rate and type of wear is the chief problem in maintaining a good mechanical seal. Since the seals are established between very hard, close-fitting surfaces without intervention of gaskets or lubricants, the sealing surfaces are subjected to wearing contact as a means of sealing. Mechanisms of failure include frictional wear which must over time abrade the seal surfaces until no seal is left. If this were the only failure mechanism, seal life could be determined by the thickness of the sealing surfaces, and parts such as pump seals could be designed with certainty as to service lives. Other factors, however, are typically limitative of seal life, causing premature seal failure, i.e. failure well before the surfaces are abraded past sealing capability. One such factor is thermal degradation. Heat caused by the unlubricated frictional engagement of the seal surfaces can reach levels destructive of the seal structure. Another factor is localized surface destruction, caused by scratching of one surface by the other, which can be exacerbated by the presence of hard particulate in the fluid being handled, with resultant leaking and seal failure. Another problem is metal transfer from one surface to the opposed surface. This phenomenon is seen in galling of the structure surfaces as portions of one surface leave and become attached to the opposite surface, producing surface defects on both structures and thus leakage and failure of the seal.
It has now been discovered that premature failure in mechanical seals, that is failure brought about by heat degradation, scratching, galling and like factors can be reduced substantially by surface diffusing the mechanical seal structures at the areas of sealing contact with an element forming an intermetallic compound with the base metal of the structure. This diffusion, suitably from a diffusion pack in accordance with well known diffusion pack technologies, produces intermetallic compounds such as cobalt or nickel aluminides, depending on the base metal and the diffusing metal, in the structure surface. The intermetallic compound modified surfaces have been found to greatly improve the mechanical seal capability of conventional mechanical structures such as those formed of Stellite alloy and cobalt base tungsten carbide. For example, the seal surfaces are galling resistant. The intermetallic compound modified structure surface is resistant to interboundary metal transfer thus effectively blocking the underlying phenomenon of galling. Moreover, surface friction is reduced, lowering temperatures at the interface of the seal parts, contributing to longer seal life and limiting thermal degradation of the seal structures. Additionally, the surface diffusion coatings have their own determinable erosion, corrosion, hardness, and other properties which can be imparted to the structure surface as desired by the primary diffusants themselves in forming the intermetallic compounds, or by added elements such as titanium for extra hardness.
It is a major object of the invention therefore to provide mechanical seals in which the seal defining structures are surface diffusion coated to form intermetallic compounds with the base metal of the structure, thereby lengthening the life and improving the effectiveness of such seals in widely different use environments. It is another object to diffusion coat mechanical seal structure surfaces to alter the response mechanism of such sealing surface to heat, erosion, corrosion and the like. It is a specific object of the present invention to provide to control metal transfer phenomena between mechanical seal surfaces and thereby control premature deterioration of the surfaces, and to increase the lubricity in mechanical seal surfaces, and thus reduce adverse effects caused or aggravated by excessive friction.
These and other objects of the invention to become apparent hereinafter are realized in a mechanical seal having a first part comprising an iron, nickel or cobalt base structure, and a relatively movable second part cooperating therewith, the first part structure having in contact with the second part in mechanical seal defining relation a surface layer consisting essentially of a diffusion coating-formed intermetallic compound of the base metal.
In particular embodiments, the second part comprises an iron, nickel or cobalt base structure, typically having in contact with the first part structure a surface layer consisting essentially of a diffusion coating-formed intermetallic compound of the base metal.
Preferably in the present mechanical seals, the intermetallic compound comprises the base metal and at least one of aluminum, boron, silicon, and carbon, and particularly the intermetallic compound comprises iron, nickel, cobalt or iron nickel aluminide, or comprises iron, nickel, cobalt or iron nickel boride. In other embodiments the intermetallic compound comprises iron, nickel, cobalt, iron nickel, silicon or boron carbide. In still other embodiments, the intermetallic compound comprises iron, nickel, cobalt or boron silicide.
Typically, in the mechanical seals according to the invention, the sealing surface layer intermetallic compound contains about 30% aluminum by weight and the balance nickel, iron or cobalt. Alternatively, the sealing surface layer intermetallic compound can contain about 10% by weight boron and the balance nickel, iron or cobalt. Preferably the surface layer comprises nickel aluminide, and may further comprise small but effective amounts of tantalum, molybdenum, chromium, zirconium, titanium or silicon diffused into the surface layer.
In particular embodiments, mechanical seal first part structure comprises a silicon or refractory metal carbide, nitride or boride and a base metal binder, such as nickel, iron or cobalt. The preferred refractory metal carbides are tungsten, tantalum, titanium or zirconium carbides, and the preferred borides tungsten, tantalum, titanium or zirconium borides.
Preferably in silicon and refractory carbide structures, the intermetallic compound comprises an aluminide of one or more of the base metals, e.g. and particularly, nickel aluminide.
Typically in the present mechanical seals the surface layer is about 0.05 millimeter in depth, and can range from 0.02 to 0.04 millimeter.
The invention further contemplates the method of forming a long wearing mechanical sealing surface on a nickel, iron or cobalt base structure which includes diffusing an intermetallic compound forming element, e.g. aluminum, into the surface of the structure from a diffusion pack e.g. an aluminizing pack under intermetallic compound forming conditions with the base metal.
Further contemplated is selecting a cobalt base tungsten carbide cermet structure or a cobalt base metal structure as the mechanical seal defining structure, and diffusing boron into the structure from a boronizing pack.